Karachi - Islamist militants stormed a naval base in the Pakistani city of Karachi late on Sunday, destroying two US-supplied surveillance aircraft, firing rockets and battling commandos sent to subdue them in one of the most brazen attacks in years, officials said.

At least four navy personnel and a paramilitary ranger were killed and 11 security forces were wounded in fighting at the Naval Station Mehran that was still ongoing on Monday morning, navy spokesperson Irfan ul Haq said.

The Pakistani Taliban claimed responsibility for the attack, saying it was part of their revenge for the US raid that killed Osama bin Laden.

It was unclear how many militants were killed or wounded. Between 10 and 15 attackers entered the high-security facility before splitting into smaller groups, setting off explosions and hiding in the sprawling facility, Haq said.

The militants attacked the base from three sides and had control of one building as of Monday morning, Interior Minister Rehman Malik said. Navy helicopters were flying over the base in search of the attackers.

"Because of the presence of several assets on the base, the operation is being carried out in a cautious, smart way," Haq said, referring to military aircraft. "That's why it's taking so long."Bin Laden raid

The co-ordinated strike rocked the country's largest city just under three weeks after the May 2 death of bin Laden in a US Navy Seals raid in the northwest garrison city of Abbottabad, an event al-Qaeda-allied extremists in Pakistan have vowed to avenge.

The unilateral American raid triggered a strong backlash against Washington, which is trying to support Pakistan in its fight against militants, as well as rare domestic criticism against the armed forces for failing to detect or prevent the operation.

The fact that militants were able to enter one of the country's largest military bases is another embarrassing blow to the Pakistani army and will raise questions over whether the attackers had inside information. That they targeted US-supplied aircraft draws attention to American aid to the military, something generals here do not talk about, fearing criticism from the county's fiercely anti-American population.

After heavy American prodding, security forces launched several operations against militants in their heartland close to the border with Afghanistan over the last three years. The extremists have struck back against police and army targets around the country.

In claiming responsibility, Pakistani Taliban spokesperson Ahsanullah Ahsan said the attack was part of their planned response to the death of the al-Qaeda chief, and that Pakistan is the top target.

The militant group hates the Pakistani government because of its alliance with the US. This is the third major attack it has claimed since the bin Laden killing, including a car bombing that slightly injured American consulate workers in the northwest city of Peshawar and a twin-suicide attack that killed around 90 Pakistani paramilitary police recruits.'Cowardly act of terror'

Pakistani Prime Minister Yousuf Raza Gilani condemned the attack, saying such a "cowardly act of terror could not deter the commitment of the government and people of Pakistan to fight terrorism."

Sunday's raid appeared to be the most serious against the military since October 2009, when militants attacked the army headquarters close to the capital, Islamabad. They held dozens hostage in a 22-hour standoff that left 23 people dead, including nine militants.

The raid began with at least three loud explosions, which were heard by people who live around the naval air station. It was unclear what caused the explosions, but they set off raging fires that could be seen from far in the distance.

An Associated Press reporting team outside the base heard at least six other explosions and sporadic gunfire.

Authorities sent in several dozen navy and police commandos to battle the attackers, who responded with gunfire and grenades, said Salman Ali, another navy spokesperson. At least two P-3C Orions, maritime surveillance aircraft given to Pakistan by the US, were destroyed, he said.

The United States handed over two Orions to the Pakistani navy at a ceremony at the base in June 2010 attended by 250 Pakistani and American officials, according to the website of the US Central Command. It said by late 2012, Pakistan would have eight of the planes.

At least one media report said a team of American technicians were working on the aircraft at the time of the strike, but US Embassy spokesperson Alberto Rodriguez said no Americans were on the base. Ali also stated there were no foreigners inside the base.

Karachi, a city of around 18 million people, has not been spared the violence sweeping the country, despite being in the south far from the northwest where militancy is at its strongest. In April, militants bombed three buses taking navy employees to work, killing at least nine people.

The Pakistani Taliban and other militant groups have little direct public support, but the army and the government have struggled to convince the people of the need for armed operations against them. The militants' identification with Islam, strong anti-American rhetoric and support for insurgents in Afghanistan resonates with some in the country.

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